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ADS5263: Offset clarification/questions

Mark Guastaferro
Genius 11095 points
Part Number: ADS5263

I think this should be in the Medical forum, but for some reason I can only select the High Speed forum.

I have a couple of questions regarding the offset spec of the ADS5263.  In the Electrical Characteristics table, it lists the Offset as +-10 mV typ, +-30mV max. Assuming fullscale is 4V diff, and 16 bit resolution, 30 mV offset results in code error of +/- 491 codes (164 codes for 10 mV offset). Is this correct?

Figure 34 shows the output code histogram for the input shorted. The main bin of the histogram is 202 LSBs less than midscale (equivalent to about 12 mV). Can this be directly related to offset? I'm not sure if this is really equivalent to the offset spec since this is measured at midscale which might include gain errors on top of offset error. 

Also, is Figure 34 measured from one device on one input? or the average of multiple inputs or devices?

Some data was collected on each input of 4 devices with no input signal in the ADC inputs (not shorted). Each ADC input is AC coupled with VCM connection through 50 ohms on each input. Collecting multiple output measurements on each ADC output at 100 MSPS across temp and averaging, the offset from midscale varied from about -140 LSBs to -460 LSBs.  Most of the variation was from ADC to ADC not so much from temp. Does this much variation make sense? Some ADCs are much higher than what Figure 34 implies. Maybe the measurement technique of leaving the inputs open is adding errors? 

  • 0
    TI__Mastermind 18275 points
    Mark,

    Your thread is assigned to the correct applications engineer. You should receive a response soon.
  • 0 in reply to Amy Schnoor
    TI__Expert 4145 points
    Hi Mark,

    The +/-30mV max offset error in the Electrical Characteristics table is what we spec for the device (~491 LSBs) across multiple ICs. We also recommend an AC short at the ADC inputs (capacitor to ground) for offset measurements.

    I believe Figure 34 is showing how small the variation in output code is for a single ADC.

    Sincerely,
    Olu
  • 0 in reply to Olu Sodipe
    TI__Genius 11095 points

    Hi Olu,

    Thanks for this information. I think I had myself confused about how a differential ADC is set-up to measure offset. 

    So to clarify, offset on the ADS5263 is measured by providing an AC short on the input which is done by connecting caps to ground on each input.  There is no direct short across the inputs, correct?

    I agree that Figure 34 is probably for one ADC in one device. Does it make sense that the offset can vary across the entire +-30 mV range when comparing the 4 ADCs within one device? Or would we expect the offsets to be a little tighter within one device?  

  • 0 in reply to Mark Guastaferro
    TI__Expert 4145 points

    Hi Mark,

    We do not recommend direct short of the inputs. Unfortunately, the data we have and that is spec'ed in the datasheet only shows variation over multiple devices and not within a single device so I can't really say.

    We can guarantee that +/-30mV spec because we test that during production for all shipped devices.

    Sincerely,

    Olu